Journal of Thermal Analysis and Calorimetry
Chemico-physical characterization and evaluation of coating properties of two
commercial organosilicons: Hydrophase and Disboxan 450
--Manuscript
Draft--Manuscript Number:
Full Title: Chemico-physical characterization and evaluation of coating properties of two commercial organosilicons: Hydrophase and Disboxan 450
Article Type: S.I. : AICAT2018
Corresponding Author: Celia Duce
Universita degli Studi di Pisa ITALY
Corresponding Author Secondary Information:
Corresponding Author's Institution: Universita degli Studi di Pisa Corresponding Author's Secondary
Institution:
First Author: Alessio Spepi
First Author Secondary Information:
Order of Authors: Alessio Spepi silvia pizzimenti Celia Duce Giovanni Vozzi Carmelo De Maria Maria Rosaria Tinè Order of Authors Secondary Information:
Funding Information: University of Pisa
(PRA2017_17) Prof. Maria Rosaria Tinè
Abstract: Two commercial organosilicons, Hydrophase, a monomeric dispersion, and Disboxan 450, an oligomeric dispersion, were studied in pure form and applied on acrylic paint replicas. Their physico-chemical characteristics, coating properties, and interaction with acrylic paint replicas were evaluated by TG, DSC, FTIR and contact angle
measurements. Hydrophase showed a higher interaction when used on the top of acrylic paint replicas than Disboxan 450. No appreciable modification was detected after two years of natural aging.
Suggested Reviewers: Ignazio Blanco iblanco@unict.it Giuseppe Lazzara giuseppe.lazzara@unipa.it Stefano Vecchio Ciprioti stefano.vecchio@uniroma1.it Ilaria Bonaduce
Chemico-physical characterization and evaluation of coating
1properties of two commercial organosilicons: Hydrophase and
2Disboxan 450
3A. Spepi 1°, S. Pizzimenti1°, C. Duce1*, G. Vozzi 2, C. De Maria2, M.R. Tiné1 4
5
1 Department of Chemistry and Industrial Chemistry, University of Pisa, Via G. Moruzzi 13, 6
56124 Pisa, Italy 7
2 “E. Piaggio” Research Center, University of Pisa, Largo Lucio Lazzarino 2, 56122, Pisa,
8
Italy 9
10
*Corresponding Author: Dr Celia Duce, e-mail: celia.duce@unipi.it.
11
°Contributed equally to this work with: Alessio Spepi, Silvia Pizzimenti 12
13
Abstract
14
Two commercial organosilicons, Hydrophase, a monomeric dispersion, and Disboxan 450, an 15
oligomeric dispersion, were studied in pure form and applied on acrylic paint replicas. Their 16
physico-chemical characteristics, coating properties, and interaction with acrylic paint replicas 17
were evaluated by TG, DSC, FTIR and contact angle measurements. Hydrophase showed a 18
higher interaction when used on the top of acrylic paint replicas than Disboxan 450. No 19
appreciable modification was detected after two years of natural aging. 20
21
Keywords: ATR-FTIR, differential scanning calorimetry, contact angle, organosilicon
22
coatings, evolved gas analysis, paint-coating interactions, polysiloxane, thermal stability,
23
thermogravimetric analysis, therm(oxy)dative degradation.
24 25 26
Manuscript Click here to access/download;Manuscript;Manuscript.doc
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62
Introduction
27
Polysiloxanes are polymers that possess both organic and inorganic properties and can react 28
with the surface, forming durable covalent bonds at the interface [1]. These bonds are 29
hydrolysable; however, they can reform easily, resulting in improved coating adhesion and 30
surface durability [1-3]. They have often been applied as restoring and conserving agents of 31
buildings and monuments [1, 2, 4]. Polysiloxanes have been used as monomers, oligomers, or 32
preformed polymers. Their handling and penetration properties are largely determined by the 33
degree of condensation as well as by the solvent. Some of them exhibit their protecting and 34
dirt repelling effects after cross-linking in situ [1-3]. Polysiloxanes used as protective coatings 35
can degrade with prolonged use in outdoor conditions [1, 5]. FTIR, and particularly ATR-36
FTIR, is often used to study the stability and modification of these coatings [6, 7]. 37
Thermal stability of various polysiloxanes has been evaluated in the literature by using 38
thermogravimetry (TG) [8-11] , thermogravimetry coupled with infrared spectroscopy (TG-39
FTIR) [12, 13, 11] and DSC [11]. The physical-chemical interaction with building materials 40
as stones and cured cement pastes has also been investigated through XPS analysis [6, 14]. 41
On the contrary, studies on the interaction with paints used in outdoor artistic murals are 42
missing. In order to fill this gap, the physico-chemical characterization and the evaluation of 43
the coating properties of two commercial hydrophobic silicone varnishes, and namely 44
Disboxan 450 (purchased from Caparol) and Hydrophase (purchased from Phase Restauro), 45
were carried out in this work by using DSC, TG coupled with FTIR, ATR-FTIR and contact 46
angle measurements. 47
Chemical formulas of Hydrophase and Disboxan 450 are unknown since they are 48
protected by patents. The only available information is that Hydrophase is a ready-to-use 49
monomeric coating agent, with no film-forming properties and a comparable molecular size 50
to water molecules (5-10 Å), while Disboxan 450 is a micro-emulsion composed of an 51
oligomeric alkyl alkoxysilane. Hydrophase penetrates deeply into the walls thus precluding 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62
the absorption of water, the damage due to ice formation, and the corrosion induced by 53
pollutants and acid rain. On the contrary, given its higher molecular size, Disboxan 450 does 54
not penetrate deep into the support and its interaction with the substrate takes place mainly on 55
the surface. Hydrophase and Disboxan 450 have been studied in the literature for their ability 56
of reducing the water penetration and the bioreceptivity of a surface. Considering the 57
hydrophobicity related to the bioreceptivity of the building materials, Urzi et al.,[15] 58
demonstrated that Hydrophase Superfici (an alkyl alkoxy silane) and Hydrophase Malte (an 59
alchyl tri-alcoxy silane) - produced by Phase Restauro are not effective when they are applied 60
alone, and need to be combined with a biocide. In the field of underwater archaeology, Crisci 61
et al. investigated capillary water absorption and simulated solar aging to evaluate 62
hydrophobic and consolidating proprieties of formulations composed of Hydrophase and a 63
biocide, applied on marble samples. After the aging cycles, the formulations with Hydrophase 64
appeared stable and showed a good resistance to exposure to solar lamp. Furthermore, the 65
capillary water test revealed that the formulations containing Hydrophase showed an 66
unpredictable further decrease in water absorption. [16] 67
The present study was carried out within the cleaning and restoration activities of the 68
acrylic wall painting “Tuttomondo” painted in 1989 by Keith Haring on the external wall of 69
the church of Sant' Antonio in Pisa. Both Hydrophase and Disboxan 450 have been applied on 70
Caparol acrylic paint replicas (the same paint brand used by K.Hering). The focus was on two 71
main points: 72
1. The interaction of Disboxan 450 and Hydrophase with Caparol acrylic paints; 73
2. The effect of two years of outdoor natural aging on coated paint samples; 74
In this perspective, the coated paint films were systematically examined after drying using 75
DSC, TG, FTIR, and contact angle measurements, before and after two years of natural 76 outdoor ageing. 77 78 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62
Experimental
79
Materials
80
Hydrophase® was purchased from Phase Restauro (Milan-Italy), and Disboxan 450® 81
from Caparol (Pisa-Italy). The acrylic colours were purchased from Caparol (Pisa-Italy). Paint 82
replicas were prepared by applying a layer of Caparol acrylic colour on the wall surface 83
miming material Capatec Putz. Both siloxane coatings were micro-sprayed onto paint 84
replicas. Disboxan was previously diluted 1:10 with water and then applied. A set made up of 85
each typology of siloxane coating/acrylic colour paint replica was naturally aged outdoors for 86 two years. 87 88 Thermogravimetry (TG) 89
A TA Instruments Thermobalance model Q5000IR equipped with a FT-IR Agilent 90
Technologies spectrophotometer model Cary 640 for Evolved Gas Analysis was used. TG 91
measurements were performed at a rate of 10 °C/min, from 50 °C to 800 °C under air flow 92
(25 ml/min). The amount of sample in each TG measurement varied between 2 and 4 mg. 93
Each experiment was performed three times. 94
TG-FTIR measurements were performed at a rate of 20 °C/min, from 50 °C to 800 °C 95
under nitrogen/air flow (90 ml/min), from 600 to 4000 cm-1 with a resolution of 4 cm-1. To 96
reduce the strong background absorption from water and carbon dioxide in the atmosphere, 97
the optical bench was usually purged with nitrogen. In addition, a background spectrum was 98
taken before the beginning of each analysis in order to zero the signal in the gas cell and to 99
eliminate any contribution from the amount of ambient water and carbon dioxide. The amount 100
of sample in each TG-FTIR measurement varied between 6 and 8 mg. Data were collected 101
using Agilent Resolution Pro version 5.2.0. 102 103 DSC 104 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62
A Perkin Elmer Pyris Diamond Differential Scanning Calorimeter was used. The 105
measurements were performed in the temperature range 30-300 °C at 10 °C/min, under air as 106
the purging gas. The sample masses ranged from 5 to 6 mg and were prepared in aluminium 107
pans. Each experiment was performed three times. 108
109
Fourier Transform Infrared Spectroscopy
110
Infrared spectra were recorded using a FT-IR Agilent Technologies Spectrophotometer 111
model Cary 640, equipped with a universal attenuated total reflectance accessory (ATRU). A 112
few micrograms of sample were used with the following spectrometer parameters; resolution: 113
4 cm-1, spectral range: 500-4000 cm-1, number of scans: 32. Spectrum software was used to 114
process the FTIR spectra. 115
116
Contact Angle
117
Optical contact angles were performed using the sessile drop technique. The 118
experimental setup for the contact angle measurements consisted in an HD webcam mounted 119
on a horizontal microscope to acquire a digital image of a drop of deionized water placed onto 120
a treated glass slide. The instrument was positioned in a room with controlled environment 121
(relative humidity of 30% and temperature of 20°C). The water was dispensed through a 122
micro-syringe and the drop volume was 2.0 ± 1.0 μL. The contact angle was obtained by 123
fitting the drop profile with the Young-Laplace model using the Drop Analysis plugin of 124
ImageJ [18]. The reported values were the average of the contact angles of three static drops 125
on different areas on the surface of the films. 126 127 128 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62
Results and discussion
129
Physico-chemical characterization of Hydrophase and Disboxan 450
130
Thermal degradation of pure Hydrophase and Disboxan has been studied by TG, DSC 131
and FTIR. The TG curve and the corresponding derivative curve, recorded under nitrogen or 132
air flow, are reported in Fig. 1. Table 1 reports the temperature of each mass loss and the
133
corresponding mass loss percentage obtained under air or nitrogen flow. 134
135
Fig. 1 TG curve of Hydrophase and Disboxan 450 (left) and the corresponding derivative (right), under nitrogen and
136
air flow at 10°C/min heating rate.
137 138
Table 1 . Experimental temperatures and the percentage mass losses of the corresponding degradation steps under air
139
and nitrogen flows of the pure coatings.
140 141 Step Hydrophase (air) Hydrophase (nitrogen) Disboxan 450 (air) Disboxan 450 (nitrogen) 1 250°C (33%) - - - 2 - - 375°C (38.8%) - 3 540°C (10%) - 471°C (shoulder) - 4 - 520°C (35.4) - 509°C (46.5%) 5 - - 622°C (7.6%) - Residual mass (700°C) 57% 64.6 % 53.6 % 53.5 % 142
The thermal decompositions of Hydrophase and Disboxan 450 were different if carried 143
out in an inert (nitrogen) or oxidative (air) atmosphere. Under nitrogen, both Hydrophase and 144
Disboxan 450 pyrolyse with a single mass loss at 509 °C and 520 °C, respectively (Fig. 1 and 145
Table 1). Under air flow, the main degradation step, corresponding to an exothermic peak in
146
DSC curve (Fig. S1 in the Supplementary Material), shifts to a lower temperature (from 520 147
°C to 250 °C for Hydrophase and from 509 to 375°C for Disboxan 450) and the TG profile 148 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62
presents a shoulder at higher temperatures (540°C for Hydrophase and 471, 622°C for 149
Disboxan 450). 150
Sun and coworkers [19] investigated the thermal degradation process, under an inert 151
argon atmosphere, of a derivative of polydimethylsiloxane (PDMS), the 152
poly(methylphenylsiloxane), and proved that it can proceed according two different 153
mechanisms: “unzip degradation” and “rearrangement degradation”. The first mechanism 154
generates cyclic siloxanes and occurs at about 400-500°C. The second, above 500°C, is 155
caused by heterolytic cleavage and rearrangement of the Si-O-Si bond in the main chain, and 156
generates low molecular weight species and cyclic siloxanes [11]. 157
In a TG-FTIR experiment carried out under nitrogen atmosphere on a sample of 158
Hydrophase, the spectrum of the volatiles evolved at 520 °C (Fig.S2) showed the presence of 159
CH4 (3015 cm-1, C-H stretching and 1304 cm-1, C-H bending), indicative of the homolytic
160
scission of the Si-CH3 bond followed by H abstraction [Error! Bookmark not defined.], and
161
of aliphatic saturated hydrocarbon compounds (between 2820 and 2960 cm-1, C-H stretching, 162
and between 1370 and 1460 cm-1, C-H bending) and unsaturated (3090 cm-1, =C-H stretching, 163
and 1652 cm-1, C=C stretching). The spectrum of the volatiles evolved, in the same
164
conditions, by Disboxan 450 at 510 °C (Fig. 2S) showed FTIR bands that are ascribable to the 165
presence, together with CH4, of cyclic siloxane compounds. Fig. 2 shows the FTIR profiles at
166
fixed wavenumbers of gases evolved with time by Hydrophase (3015 cm-1, C-H stretching of
167
CH4) and Disboxan 450 (1090 cm-1, Si-O stretching vibration of cyclic siloxane compounds).
168
Under air flow, above 350°C, the thermo-oxidative degradation of Hydrophase 169
produces carbon dioxide and water (indicating the combustion of the organic part of the 170
sample), while Disboxan 450 (Fig. S3), in addition to the peaks typical of carbon dioxide and 171
water, shows peaks ascribable to the presence of siloxane oligomers (bands between 810 and 172
1260 cm-1). These results suggest that Hydrophase degrades with the simplest pathway 173
(homolytic Si-CH3 breakage), while Disboxan 450 has a typical PDMS derivative pathway,
174 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62
which occurs with “rearrangement degradation” leading to the formation of cyclic siloxanes 175
[11, 20], thus confirming the monomeric nature of Hydrophase and oligomeric nature of 176
Disboxan 450. [21] 177
178
Fig. 2 Evolved gas analysis profiles under nitrogen flow in function of time at fixed wavenumbers of
179
Hydrophase (3015 cm-1, C-H stretching of CH4) and Disboxan 450 (1090 cm-1, ν Si-O of cyclic siloxane
180
compounds).
181 182
These results are supported by the FTIR-ATR spectra of Hydrophase and Disboxan 450 183
reported in Fig. 3 (and Table S2 for the main FTIR absorptions and the corresponding 184
vibrational assignments). 185
186
Fig. 3 FTIR-ATR spectra in the range 4000-600 cm-1 - Hydrophase (left), and Disboxan 450 (right).
187 188
The FTIR-ATR spectra of both coatings show the peaks typical of methyl siloxane 189
derivatives. The absorption bands of the alkylic portion of the samples can be totally ascribed 190
to the methyl groups present. Note that the OH stretching band (3402 cm-1) is present only in 191
Hydrophase samples and that the absorption bands related to Si-O vibrations are different for 192
Hydrophase compared to Disboxan 450. In particular, the νs and νas Si-O of Disboxan 450 are
193
at 1092 and 1020 cm-1 respectively, while those of Hydrophase at 1065 and 902 cm-1. This 194
could be due to the monomeric nature of Hydrophase. In fact, when silanol (Si-OH) groups 195
are present in the siloxane structure, the absorption of Si-O bond shifts to a lower 196
wavenumber [22]. 197
Hydrophase and Disboxan 450 coating properties
198
In order to investigate the coating properties of Hydrophase and Disboxan 450 and their 199
interactions with the acrylic paints, paint/Hydrophase and paint/Disboxan 450 replicas were 200
prepared according to the procedure described in the experimental section and studied by TG, 201
DSC, FTIR-ATR, and contact angle measurements. 202 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62
The Caparol acrylic color is composed of a styrene/n-butylacrylate copolymer [17]. 203
Table 2 reports the temperatures and the mass losses, under air flow, of the acrylic paint alone
204
or coated with Hydrophase or Disboxan 450. The same quantities for the two pure coatings 205
are reported in Table 1. Error! Reference source not found. (left) shows the TG curves under air
206
flow, and the corresponding derivative curves, of samples of acrylic paint and of acrylic paint 207
covered with Hydrophase. The thermograms of the acrylic paint alone show the typical 208
behaviour of these copolymers [23], with a first small mass loss in the range 25-260 ° C 209
(shoulder at 220°C in the derivative curve), the main mass loss at 347°C, due to the 210
degradation of the acrylate unit, and a third decomposition step in the range 390–500 °C 211
(shoulder at about 417°C), due to the degradation the styrene unit. The DSC curve of the 212
acrylic paint (Error! Reference source not found. – right) reveals the presence of three 213
exothermic effects in the temperature range 25-500 °C, which are related to the main 214
decomposition phases highlighted by thermogravimetry. 215
216
Fig. 4 TG curve and its derivative of the acrylic paint, alone and covered with Hydrophase, performed under air flow
217
at 10°C/min heating rate (left). DSC curve of the acrylic paint alone and covered with Hydrophase under air flow at
218
10°C/min heating rate (right).
219 220
Table 2 Experimental temperature and mass loss percentage of thermal degradation steps under air of acrylic paint
221
alone or covered by Hydrophase and Disboxan 450.
222 223
N° step Acrylic paint Acrylic paint + Hydrophase Acrylic paint + Disboxan 450 1 2 25-260°C (4.1%) 347°C (20.5%) 25-299°C (4.6%) 374°C (12.9%) 25-260°C (3.7 %) 345°C (17.5%) 3 390-500°C (4.1%) 390-500°C (2.4%) 360-500 °C (6.3%) Residual mass (500°C) 71.2% 80.1 % 72.4 % 224
The TG analysis of the acrylic paint coated with Hydrophase still shows three 225
degradative steps, but they occur at higher temperatures. The first step (at about 270 °C), is 226
only partially due to the degradation of the acrylic paint and can be mainly attributed to the 227 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62
main decomposition step of Hydrophase (250°C), the second step, with the main mass loss, is 228
shifted at 374°C while the third step is shifted at about 450°C. (Error! Reference source not 229
found. left, Error! Reference source not found.). An analogous behaviour can be observed 230
in the DSC curves, with the strengthening of the exothermic effect at about 250°C due to 231
thermoxidation of Hydrophase, and the shift to higher temperatures of the two remaining 232
peaks (Error! Reference source not found. right, Error! Reference source not found.). These 233
results demonstrate an effective interaction between the acrylic resin and Hydrophase and 234
highlight the greater stability of the coated paint. On the contrary, both the DSC thermogram 235
and the TG profile of acrylic paint are practically unaffected by the coating with Disboxan 236
450 (See Fig. S4). 237
The FTIR-ATR spectrum of the acrylic paint alone is reported in Fig. S5. The 238
interactions of Hydrophase and Disboxan 450 with acrylic paints have been investigated also 239
by FTIR-ATR measurements. The spectra of the acrylic paint coated with Hydrophase (Fig. 240
S6) clearly show a decrease in the OH stretching absorbance at about 3300 cm-1 and an 241
increase in the Si-O stretching absorbance in the range 1000-1120 cm-1 (Fig. S6). These 242
evidences prove the reduction in the free OH content due to the corresponding formation of a 243
bond between siloxanes and acrylic paint and explain the increased thermal stability of the 244
coated samples. Moreover, the Hydrophase coating causes an increase in the C-H stretching 245
absorbance (2870-2965 cm-1) on the spectra of acrylic paint replicas due to the presence of
246
alkyl groups bonded to Si atoms of the coating. 247
Error! Reference source not found. shows the FTIR spectra of fresh paint replicas 248
coated with Hydrophase compared to those naturally aged outdoors for two years. The FTIR 249
spectra of the fresh and aged paint coated with Disboxan 450 are reported in Fig.S6. 250
Hydrophase coated paint replica is less stable over time than that coated with Disboxan 450. 251
Its decomposition can lead to the breakage of Si-C bonds and the formation of CH4 and SiO2
252
through radical reactions catalysed by light or by heat during outdoor exposure. As a result, 253 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62
absorption decreasing in the range 2830-2960 cm-1 (C-H bonds) and in the range 900-1100
254
cm-1 (Si-O bonds) are observed. Regarding the stability of Disboxan 450, Wang et al., [24] 255
attributed the ageing resistance of this coating to the inability of the ultraviolet rays to break 256
the siloxane bond (Si-O-Si). 257
258
Fig. 5 FTIR spectra of Hydrophase coated paint fresh and 2 years naturally outdoor aged.
259 260
Commercial siloxanes are commonly used as water repellent coatings for wall paints, 261
for this reason, it is important to study the wettability of the coated surface and the stability of 262
the coating over time. The wettability of the two coatings was studied by contact angle 263
measurements. Contact angle value is a simple and a powerful parameter suitable to 264
characterize three-phase junctions, and thus to determine the wettability of surfaces with 265
respect to a specific liquid. Sessile drop is today the most widely used method to measure this 266
parameter [18]. 267
Contact angle values can be affected by various factors: surface roughness, morphology, 268
temperature, heterogeneity and reactivity of surface with water drops [6]. For this reason, we 269
made some assumptions which allowed us to determine and interpret these data: 270
1) all the samples have the same roughness, so the contact angle values are 271
comparable; 272
2) all the samples are homogeneous, or the actual heterogeneity is the same in all the 273
samples; 274
3) there is no swelling, absorption, spreading or evaporation of water droplets in the 275
time frame of the analysis; 276
4) all the measurements have been collected in the same temperature conditions and 277
the effect of temperature changes is negligible; 278
5) the Young-Laplace equation gives the best value of the equilibrium contact angle. 279 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62
On the basis of these assumptions, and in line with the widely accepted theories, the 280
contact angle values, measured for the pure acrylic colours, suggested a low hydrophilicity (θ 281
≈ 60°) of the acrylic paint sample. The coating with both Hydrophase and Disboxan 450 282
modify the surface properties. The paint replicas coated with Disboxan 450 showed an 283
increase in contact angle value (θ ≈ 95°) and the surface became hydrophobic (Fig. 6). On the 284
other hand, the paint replicas coated with Hydrophase showed only a small increase in contact 285
angle (θ ≈ 68°) and this indicates that the treatment with Hydrophase promotes only small 286
changes in the wettability of the surface which maintains its hydrophilicity. 287
Aging promotes only small changes in the surface wettability, thus confirming the 288
stability of the two coatings over time. Both naturally-aged coated paint replicas showed a 289
small decrease in contact angle value (Disboxan 450 coated samples, θ ≈ 92°; Hydrophase 290
coated samples, θ ≈ 64°) and this indicates an increase in the polarity and hydrophilicity of the 291
film surface, due to the action of pollutants, light and rain. After two years of natural outdoor 292
ageing, the Hydrophase coated samples shows a contact angle similar to the fresh sample, 293
with a low hydrophilicity, while, in the Disboxan 450 coated samples, the surface remained 294
hydrophobic with θ > 90°. 295
296
Fig. 6 Water droplet images captured immediately after droplet deposition on blue paint replicas (A), on paint replica
297
coated with fresh Disboxan 450 (B) and on blue paint replicas coated with Disboxan 450 after two years of natural
298 outdoor ageing (C). 299 300
Conclusions
301This work provides the physical chemical characterization of two commercial 302
polysiloxanes, Hydrophase and Disboxan 450, commonly used as water repellent coatings for 303
wall paints and artistic wall murals and highlights some differences in the behaviour of the 304
two coatings which can be useful to the restorers in the choice of the best solution. 305
Hydrophase coating is less stable in air than Disboxan and its thermal degradation 306
occurs with a simple mechanism, due to the homolytic scission of the Si-CH3 bond followed
307 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62
by H abstraction. Differently, the Disboxan decomposes through heterolytic cleavage and 308
rearrangement of the Si-O-Si bond in the main chain. This different behavior may be 309
attributed to the monomeric nature of Hydrophase while Disboxan is an oligomeric 310
polysiloxane. On the other hand, Hydrophase interacts with the underlaying acrylic paint 311
more strongly than Disboxan. In fact, the FTIR spectra clearly show a decrease in the 312
intensity of the OH stretching absorbance (3300 cm-1) in the acrylic paint replicas when they
313
are coated with Hydrophase or Disboxan 450, more marked in the case of Hydrophase This 314
decrease confirms the interaction between the coating siloxanes and the acrylic colour that 315
leads to the formation of a Si-O bond and the consequent reduction of the free OH content. 316
However, Disboxan 450 does not induce any modifications in the thermal stability of the 317
acrylic support while the interaction of Hydrophase, causes an increase in the resin’s thermal 318
stability with the shifts of the decomposition temperatures to higher values. Moreover, upon 319
two years of natural aging, FTIR analyses reveals that Hydrophase coating is less stable over 320
time compared to the oligomeric Diboxan coating. Contact angle measurements show that the 321
coating with Disboxan 450 increases the hydrophobicity of the paint surface, while 322
Hydrophase promotes only small changes in the wettability of the surface which maintains its 323
hydrophilicity. Both replicas of naturally aged coated paint showed only a small decrease in 324
the value of the contact angle, indicating good resistance to the action of pollutants, light and 325
rain. 326
Acknowledgements
327
This work was supported by PRA_2017_17 funded by University of Pisa
328
References
329 330
1. Witucki GL. A silane primer: chemistry and applications of alkoxy silanes. Journal of 331
coatings technology. 1993;65:57-. 332
2. Tsakalof A, Manoudis P, Karapanagiotis I, Chryssoulakis I, Panayiotou C. Assessment of 333
synthetic polymeric coatings for the protection and preservation of stone monuments. J Cult 334 Herit. 2007;8(1):69-72. 335 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62
3. Eduok U, Faye O, Szpunar J. Recent developments and applications of protective silicone 336
coatings: A review of PDMS functional materials. Prog. Org. Coat. 2017;111:124-63. 337
doi:https://doi.org/10.1016/j.porgcoat.2017.05.012.
338
4. Kahraman MV, Kuğu M, Menceloğlu Y, Kayaman-Apohan N, Güngör A. The novel use of 339
organo alkoxy silane for the synthesis of organic–inorganic hybrid coatings. J. Non-Cryst. 340
Solids. 2006;352(21-22):2143-51. 341
5. Li D, Xu F, Liu Z, Zhu J, Zhang Q, Shao L. The effect of adding PDMS-OH and silica 342
nanoparticles on sol–gel properties and effectiveness in stone protection. Appl. Surf. Sci. 343
2013;266:368-74. 344
6. Fermo P, Cappelletti G, Cozzi N, Padeletti G, Kaciulis S, Brucale M et al. Hydrophobizing 345
coatings for cultural heritage. A detailed study of resin/stone surface interaction. Appl. Phys. 346
A. 2014;116(1):341-8. 347
7. Brachaczek W. Comparative analysis of organosilicon polymers of varied chemical 348
composition in respect of their application in silicone-coating manufacture. Prog. Org. Coat. 349
2014;77(3):609-15. 350
8. Thomas TH, Kendrick T. Thermal analysis of polydimethylsiloxanes. I. Thermal 351
degradation in controlled atmospheres. Journal of Polymer Science Part A‐ 2: Polymer 352
Physics. 1969;7(3):537-49. 353
9. Jovanovic JD, Govedarica MN, Dvornic PR, Popovic IG. The thermogravimetric analysis 354
of some polysiloxanes. Polym. Degrad. Stab. 1998;61(1):87-93. 355
10. Deshpande G, Rezac ME. The effect of phenyl content on the degradation of poly 356
(dimethyl diphenyl) siloxane copolymers. Polym. Degrad. Stab. 2001;74(2):363-70. 357
11. Tomer NS, Delor-Jestin F, Frezet L, Lacoste J. Oxidation, chain scission and cross-linking 358
studies of polysiloxanes upon ageings. Open Journal of Organic Polymer Materials. 359
2012;2(02):13. 360
12. Camino G, Lomakin S, Lazzari M. Polydimethylsiloxane thermal degradation Part 1. 361
Kinetic aspects. Polymer. 2001;42(6):2395-402. 362
13. Camino G, Lomakin S, Lageard M. Thermal polydimethylsiloxane degradation. Part 2. 363
The degradation mechanisms. Polymer. 2002;43(7):2011-5. 364
14. Stewart A, Schlosser B, Douglas EP. Surface modification of cured cement pastes by 365
silane coupling agents. ACS applied materials & interfaces. 2013;5(4):1218-25. 366
15. Urzì C, De Leo F. Evaluation of the efficiency of water-repellent and biocide compounds 367
against microbial colonization of mortars. Int. Biodeterior. Biodegrad. 2007;60(1):25-34. 368
doi:https://doi.org/10.1016/j.ibiod.2006.11.003.
369
16. Crisci GM, La Russa MF, Macchione M, Malagodi M, Palermo AM, Ruffolo SA. Study 370
of archaeological underwater finds: deterioration and conservation. Appl. Phys. A. 371
2010;100(3):855-63. doi:10.1007/s00339-010-5661-9. 372
17. La Nasa J, Orsini S, Degano I, Rava A, Modugno F, Colombini MP. A chemical study of 373
organic materials in three murals by Keith Haring: A comparison of painting techniques. 374
Microchem. J. 2016;124:940-8. doi:https://doi.org/10.1016/j.microc.2015.06.003. 375
18. Stalder AF, Melchior T, Müller M, Sage D, Blu T, Unser M. Low-bond axisymmetric 376
drop shape analysis for surface tension and contact angle measurements of sessile drops. 377
Colloids Surf. Physicochem. Eng. Aspects. 2010;364(1):72-81. 378
doi:https://doi.org/10.1016/j.colsurfa.2010.04.040.
379
19. Sun JT, Huang YD, Cao HL, Gong GF. Effects of ambient-temperature curing agents on 380
the thermal stability of poly(methylphenylsiloxane). Polym. Degrad. Stab. 2004;85(1):725-31. 381
doi:https://doi.org/10.1016/j.polymdegradstab.2004.03.018.
382
20. González-Rivera J, Iglio R, Barillaro G, Duce C, Tinè M. Structural and Thermoanalytical 383
Characterization of 3D Porous PDMS Foam Materials: The Effect of Impurities Derived from 384
a Sugar Templating Process. Polymers. 2018;10(6):616. 385 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62
21. Colombini M, Modugno F, Di Girolamo F, La Nasa J, Duce C, Ghezzi L et al. Keith 386
Haring and the city of the Leaning Tower: preservation of the mural" Tuttomondo". 2015. 387
22. Hofman R, Westheim JGF, Pouwel I, Fransen T, Gellings PJ. FTIR and XPS Studies on 388
Corrosion-resistant SiO2 Coatings as a Function of the Humidity during Deposition. Surf. 389
Interface Anal. 1996;24(1):1-6. doi:doi:10.1002/(SICI)1096-9918(199601)24:1<1::AID-390
SIA73>3.0.CO;2-I. 391
23. de la Fuente JL, Fernández-García M, López Madruga E. Characterization and thermal 392
properties of poly(n-butyl acrylate-g-styrene) graft copolymers. J. Appl. Polym. Sci. 393
2001;80(5):783-9. doi:doi:10.1002/1097-4628(20010502)80:5<783::AID-394
APP1155>3.0.CO;2-5. 395
24. Wang ZY, Liu FC, Han EH, Ke W. Ageing resistance and corrosion resistance of silicone-396
epoxy and polyurethane topcoats used in sea splash zone. Mater. Corros. 2013;64(5):446-53. 397 doi:doi:10.1002/maco.201106269. 398 399 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62
Supplementary Information
Click here to access/download
Supplementary Material
Supplementary Information.doc
Cover Letter
